LAGOS, (Reuters) - The final stage of Nigeria's election began on Saturday with voting for powerful state governors, two weeks after a presidential poll saw an incumbent leader unseated at the ballot box for the first time.

The 36 governors are among the most influential politicians in Nigeria, Africa's biggest oil producer and top economy, with budgets larger than those of small nations and influence that can decide who goes on presidential tickets.

With so much at stake, candidates in past governorship elections have often played dirty, snatching ballot boxes, manipulating turnout and engaging in thuggery and intimidation.

Voting began in most states of the most populous African state by midmorning but sporadic violence in coastal Rivers state, Nigeria's oil hub, disrupted balloting in many districts.

Two people were killed in election-related incidents. A policeman in Port Harcourt was hacked to death on Friday night, Rivers police spokesman Ahmed Mohammad said. In Kebbi state in the northwest, gunmen killed an opposition party agent and wounded three others, police and hospital sources said.

Muhammadu Buhari of the All Progressives Congress (APC) beat President Goodluck Jonathan of the People's Democratic Party (PDP) last month with 15.4 million votes to 13.3 million.

The vote, which Buhari won on pledges to clean up Nigeria's notoriously corrupt politics and crack down harder on the Islamist Boko Haram insurgency engulfing the northeast, was deemed free and less violent than past polls.

Yet for many Nigerians, who their governor is matters more than who sits in the capital Abuja. In the early going of Saturday's vote, however, turnout appeared lower than for the presidential poll due to apathy in the south and southeast after the PDP defeat on March 28 and fears of violence.

â"Many people have not come out to vote...They lost interest in the election because of the outcome of the presidential," Ifechukwu Onyema, a voter in southern Anambra state, said.

INCIDENTS DISRUPT SOME VOTING

Early on Saturday, police fired shots into the air to clear an angry mob of 2,000 APC members and sympathisers in the Rivers state capital Port Harcourt, a Reuters witness said.

Shooting on the streets of two Rivers towns, Abonnema in the Akuku-Toru region and Buguma in Asari-Toru, forced voters to remain indoors, eyewitnesses and an observer said. The electoral commission office was set on fire in Abonnema and vehicles meant to distribute voting materials were burnt in Buguma.

In the northern state of Kaduna, a police spokesman said security forces fired tear gas at youths who tried to divert a vehicle transporting electoral materials in the Danmusa district, delaying the start of the vote.

Legislative elections also held on March 28 shifted power away from the PDP, which has run Nigeria since the end of military rule in 1999, to the APC, and left Buhari's party with a majority in both houses of parliament.

The APC sought on Saturday to build on its gains, while the PDP hoped to claw back some power, especially in two battlegrounds -- the megacity of Lagos, Nigeria's economic engine generating up to a third of its GDP, and Rivers.

Outgoing Lagos APC governor Babatunde Fashola is credited with transforming the metropolis of 21 million people and an economy twice the size of Kenya's with infrastructure projects, although he has also been criticised for slum clearance.

The new governors will take office in 29 states on May 29, as will Buhari.

Biometric ballot cards distributed to 56.7 million people were used for the first time in last month's vote, and helped prevent fraudulent practices like multiple voting or ballot box stuffing. They were again being used on Saturday.

"The card reader is the only way that rigging on a large scale can be stopped in this country," Abubakar Momoh, of the Independent National Electoral Commission (INEC), told Reuters.